1. Field of the Invention
This invention relates to an abrasive composition. More particularly, it relates to an abrasive composition used for polishing a substrate for a magnetic recording medium to give a magnetic recording disc having minimized surface irregularities, above which a magnetic head is capable of floating at a low flying height.
2. Description of the Related Art
Magnetic recording discs, i.e., memory hard discs, are widely used as memory means giving quick access to information as external memories of computers and word processors. The magnetic recording discs are made by a process wherein a substrate composed of an aluminum alloy disc blank with a nickel-phosphorus (NiP) surface coating, formed by electroless plating, is surface-polished, and then, a chromium underlayer, a cobalt alloy magnetic film and a carbon protective film are formed in this turn on the polished NiP surface by a sputter deposition.
If the substrate has irregularities on the polished surface which have a height larger than the flying height of a magnetic read/write head, when the magnetic head floating above the magnetic disc flies at a high speed, the head impinges upon the irregularities with the result of damage of the magnetic head. If the substrate for the magnetic recording disc has polish marks or irregularities, the marks or irregularities develop on the surfaces of the chromium underlayer, the cobalt alloy magnetic film and the carbon protective film in turn, and consequently a functional defect is caused on the magnetic recording disc. Thus it is necessary to polish the substrate surface with a high precision for giving a magnetic recording disc having a smooth surface.
A wide variety of abrasive compositions and polishing methods have heretofore been proposed for removing or minimizing irregularities on the surface of substrate for magnetic recording discs and providing a smooth substrate surface with neither polish marks nor irregularities. As examples of the proposed abrasive compositions or polishing methods, there can be mentioned (1) a two-stage polishing method using an abrasive composition comprising aluminum oxide having incorporated therewith (i) an oxidizing agent such as sodium hypochlorite, or colloidal aluminum, and (ii) cerium dioxide (Japanese Unexamined Patent Publication hereinafter abbreviated to "JP-A"! 60-108489), (2) an abrasive composition comprising alumina having incorporated therein sulfumic acid or phosphoric acid (JP-A 61-291674), (3) an abrasive composition comprising alumina having incorporated therein aluminum nitrate (JP-A 62-25187), (4) an abrasive composition comprising alumina having incorporated therein boehmite (JP-A 1-188264), (5) an abrasive composition comprising alumina having incorporated therein a metal salt and boehmite (JP-A 1-205973), (6) an abrasive composition comprising alumina having incorporated therein a metal sulfite salt (JP-A 2-158682), (7) an abrasive composition comprising alumina having incorporated therein boehmite and an ammonium salt of an organic or inorganic acid (JP-A 2-158683), (8) a polishing method using an abrasive composition comprising an alumina slurry having pre-treated with an ultrasonic filter (JP-A 3-106984), (9) an abrasive composition comprising alumina having incorporated therein boehmite and a water-soluble peroxide (JP-A 3-115383), (10) an abrasive composition comprising alumina having incorporated therein an amino acid (JP-A 4-108887), (11) an abrasive composition comprising alumina having incorporated therein (i) aluminum sulfate and/or aluminum chloride, and (ii) at least one compound selected from peroxide, nitric acid, a nitrate salt, a nitrite salt and an aromatic nitro compound (JP-A 4-275387), (12) an abrasive composition comprising alumina having incorporated therein a chelate compound, boehmite and an aluminum salt (JP-A 4-363385), (13) an abrasive composition comprising alumina having incorporated therein boehmite which has been obtained by heat-treating rectangular primary particles of gibbsite, and (14) an abrasive composition comprising colloidal silica particles and a chemical etching agent (JP-A 7-240025).
In the above-mentioned proposals (1) through (13), finely divided alumina or other aluminum compounds having an average particle diameter of about 1 .mu.m are used as an abrasive, and thus, the substrate surfaces polished by these abrasive compositions exhibit a smoothness of an extent such that a practically acceptable flying height can be ensured. However, in these years, a lower flying height is eagerly desired for allowing recording density to be increased, and it is difficult or impossible by these proposals to provide a perfect substrate surface with a high precision on which the desired lower flying height can be achieved.
In the above-mentioned proposal (14), colloidal silica particles having an average particle diameter of several-tens nanometer are used as an adhesive, a polished substrate surface with a high precision can be obtained. However, the rate of polishing is low and thus the productivity is low, and, when polishing is conducted for a long time, the circumferential edge part of a substrate surface is undesirably abraded in excess.
When an abrasive with a high hardness is used for polishing, polish marks tend to occur on a substrate surface, and thus, an abrasive with an appropriate hardness should be chosen depending upon the particular hardness of the NiP coating on the substrate. Further, in view of the quality (such as absence of polish marks and reduced number of pits) required for an aluminum magnetic recording disc of the type above which a magnetic head floats at a low flying height, an abrasive having a particle diameter of submicrons and a sharp particle size distribution should be chosen.
In general an .alpha.-alumina is used as abrasive for an aluminum magnetic recording disc. However, .alpha.-alumina has a very high hardness as compared with that of the NiP coating, and therefore, when .alpha.-alumina is used as an abrasive for an aluminum disc of the type floating at a low flying height, the crystalline structure, the particle size distribution and other properties of the .alpha.-alumina abrasive should be strictly controlled. Further, finely divided alumina particles have a problem such that, although grinding or pulverizing is employed for the industrial production thereof, the yield of particles of the desired size is low and thus their productivity is low.
In contrast to .alpha.-alumina abrasive, a silica abrasive has a low hardness and does not cause polish marks and pits on the substrate surface. However, the rate of polishing is very low and the productivity is low.